Economics and environmental necessity is driving the paper industry to increase their use of recycled fibers. Among the advantages of using recycled paper are cost savings, less refinement, easy availability, and environmental sustainability. However, increasing the number of passes through the recycling process makes the recycled fibers progressively shorter and more rigid resulting in decreases in pulp strength and also fiber-fiber bonding strength. Strength is an integral parameter for packaging grade paper. One way to recover the strength lost during the recycling process is through further refinement, but in case of recycled fibers the refining process has very limited window. Another approach is the utilization of strength chemicals during the paper making process. However, interference with recycled additives, uniform formation of paper, drainage, and productivity are the major factors that need to be taken into account.
Additives are added to the pulp slurry prior to sheet formation to improve machine runnability, productivity, and paper properties. These include wet and dry strength additives (e.g. cationic and anionic polyacrylamides, functionalized polyamides with epihalohydrin, polyvinylamines), retention and drainage aids (e.g. alum, polyethylenimines), defoamers, fillers that control pitch and stickies. Various sizing agents, such as rosin, alkyl ketene dimer (AKD), or alkyl succinic anhydride (ASA) that impart hydrophobic properties are also added.
Common wet end additives for enhancing dry strength of the finished sheet are starch or guar gum. The modification of starch to improve its performance has been well documented. The choice and type of the starch for strength application varies from region to region and is dependent on its cost and availability. (See “Starch and Starch Products in Paper Coating,” R. L. Kearney and H. W. Maurer, Ed. 1990).
Cross-linking starch to improve its strength enhancing properties has been employed. For example, Huang et al. describe the use of boron containing compounds to crosslink starch during the paper making process, resulting in improved physical and mechanical properties (WO 2004/027,149 A1). The cross-linked starch composition comprises a reaction product formed by reacting starch slurry with boric acid or zinc borate during the gelatinization process.
U.S. Pat. No. 6,303,000 granted to Floyd et al., discloses gelatinized starch compositions cross-linked with a glyoxal resin and the use of same in papermaking. During the starch gelatinization process, starch is reacted with blocked glyoxal which results in starch cross-linking and this mixture is added to the pulp slurry before sheet formation. The limitation of these particular starch cross-linking compounds is that compression strength can be improved, whereas resistance to puncture tends to suffer.
Strength additives that are added at the wet end to improve paper strength, especially in paper made from recycled fibers, include amphoteric acrylamide based polymers and coacervate technology. An example of the former is described in U.S. Pat. No. 5,698,627, issued to Oguni, which teaches the synthesis of acrylamide based amphoteric copolymers that improve freeness, retention, and dry strength of recycled corrugated base paper. An example of coacervate technology is described in U.S. Pat. No. 6,294,645. This wet end dry strength system is comprised of a low charge polyamidoamine-epichlorohydrin and an anionic polyacrylamide, added sequentially to a pulp slurry.
Polyvinylamine has been utilized as a dry and wet strength additive, and a retention and drainage aid in the papermaking process. Due to high density of amine functionality, this polymer possesses higher charge density and ultimately has enhanced hydrogen bonding between cellulose fiber and the polymer chain. Weisgerber et al. in U.S. Pat. No. 2,721,140 disclose the use of polyvinylamine, prepared by the hydrolysis of polyvinyl N-phthalimide, as a wet strength additive for paper making. U.S. Pat. No. 5,961,782 issued to Luu et al., discloses use of polyvinylamine to make crosslinkable creping adhesive formulations. Niessner et al, in U.S. Pat. No. 6,159,340, discloses the use of polyvinylamine as dry and wet strength additives in paper and paperboard production. U.S. Pat. Nos. 4,421,602, 6,616,807 and 6,797,785 disclose use of polyvinylamines as drainage aids, flocculants, and retention aids in the paper making process.
Interference with excess anionic trash coming via recycling process coupled with poor fiber quality demands significantly increased additive levels. In addition to high cost, wet end additives reach a plateau performance, that is, further chemical does not provide increased performance. Practitioners of paper making have overcome these limitations by applying additives after paper formation. Employed techniques include metered size press, puddle size press, spray, roll coater, blade water, and air knife coater. Coating or surface sizing additives commonly employed are polyacrylic emulsions, poly(styrene-co-butadiene) emulsions with various particle sizes, poly(vinylacetate), and polyvinyl alcohol. Because these additives have good film forming properties, they typically are used to impart a certain amount of resistance to various liquids. An example of this approach is provided in European Patent 1,824,937.
The most commonly applied surface additive is starch. Excess use of starch may have negative impacts on other paper properties, like fold cracking, and also productivity, for example higher energy drying. Since size press application of starch is an on-machine operation, any problem which interferes with operation of the size press has the potential to interfere with operation of the entire paper machine.
The amount of starch that can be applied during size press treatment of a paper sheet (i.e., the wet pick up of the sheet) is dependent on the size press conditions, the viscosity of the starch solution and the penetration of starch solution into the paper sheet (U.S. Pat. No. 4,191,610 issued to Prior). Modification of size press starch usually introduces functionality to the starch molecule, and can decrease its viscosity. The maximum amount of modified starch that can be economically and practically applied during size press treatment is about 10 g/m2 and for unmodified or native or high viscosity starch the maximum is 5 g/m2. U.S. Pat. No. 5,242,545 discloses running the size press treatment at higher temperature and increasing size press nip pressure, the starch loading can be increased to 17-20 g/m2. Further, WO 2006/027,632 A2 teaches that lignosulfonate, contained in the black liquor waste stream of a typical pulping process, can be mixed with starch to lower the size press solution viscosity and allows application of a higher solid content on paper during size press treatment, helping to conserve energy during sheet drying process.
U.S. Pat. No. 7,217,316 teaches a process of oxidizing protein flour sourced from plants to produce a coating composition for improving strength in paper. While the strength performance of the disclosed composition shows improved efficiency over starch or oxidized starch, levels of the coating composition described for said effect are 6-12 g/m2.
U.S. Pat. No. 5,281,307 issued to Smigo, discloses the use of vinylalcohol and vinylamine copolymer crosslinked using glyoxal for dry end application. The submerging of Whatman filter paper in polymer solution and subsequent drying shows improvement in paper properties.
Of the aforementioned wet end strength additives many have found utility at the dry end. Whereas the components of a wet end strength system are added sequentially to a slurry of pulp, which moderates potential incompatibilities between components, each component of a surface applied strength additive system needs to be combined into a single, stable solution. As starch is almost always a component of a dry strength system, other components need to exhibit solution stability when combined with starch, i.e. no precipitation, no gel formation, or severe viscosity increases. Glyoxal releasing chemicals and glyoxal containing polymers would be expected to be compatible as their cross linking activation typically requires temperatures not encountered until the drying section of a paper machine (U.S. Patent Application 2005/0161182 A1). Another example of cross linking dry strength additives is disclosed in U.S. Patent application 2009/0020249 wherein the surface application of poly(acrylic acid) with inorganic substances, e.g. zinc oxide, is described. U.S. Patent Application 2005/0287385 discloses styrene-butadiene latex coating composition that provides enhanced compression strength when applied to already formed, substantially dry paper. U.S. Pat. No. 7,482,417 discloses a surface applied dry strength agent comprised of an amphoteric acrylamide copolymer. When the surface applied starch is anionic, combination with highly cationic, dry strength agents, for example polyvinylamine, a precipitate, or gel may form.